It has wisely been said that there's no percentage in arguing with an idiot, because he'll drag you down to his level, then beat you with experience. The same holds true for those insincere sorts who aren't really interested in an answer to their loaded question. So I distracted the fellow with some version of "Look! Over there. Shiny." And I went on with my day.
The fact is, though, that if sincerely asked, it is a fair question.
I could direct you to Hugo Grotius, and that might help. Grotius' life spanned the end of the Sixteenth Century and the beginning of the Seventeenth – a time of constant, practically universal war. Nations all across Europe were sending their finest sons ahead of columns of their peasants, augmented by cadres of mercenaries, to kill the fine sons, peasants and mercenaries of their neighbors. The Portuguese were killing and being killed by the Spanish. The English were (as always) killing their Irish neighbors, along with any Spaniards they could find. The Russians were killing the Swedes. Nearly everyone in Europe was killing Turks. There were various wars of succession and countless revolts, uprisings and rebellions. Most notably, the last 27 years of Grotius' life were the first 27 years of the Thirty Years War.
Essentially beginning as a religious war that pitted most of the Roman Catholic nations of Europe against most of the Protestant ones, then devolving into a continental melee where more or less everyone was seeking to seize power and territory from more or less everyone else, the Thirty Years War is unparalleled in history for its savagery. The war killed 8 million people at a time when the population of Europe probably didn't much exceed 50 million. Civilian populations were targeted for extermination. Plague, pillage and rape were common to the orders of battle.
And it was against this backdrop of chaos and misery (and while he was himself fleeing across Europe to avoid religious persecutors who had jailed him and threatened worse) that Grotius produced De jure belli ac pacis (On the Law of War and Peace). You can read it here in English, or buy it in Latin to get the full flavor of the work. Grotius was a Christian. And when he wasn't trying to set down a rational rule book for how men might conduct war righteously, he was writing one the first widely published systemic defenses of Protestant Christianity. You can read De veritate religionis Christianae for free online, if you have the Latin and the inclination.
So, yes, I could direct you to Grotius, who knew what I know: that the command that a Christ follower should "turn the other cheek," did not mean that a Christian ought to stand by while the greatest gift besides salvation, the very gift of life, is threatened by evil. Grotius knew that Jesus' proscription was against seeking revenge or taking offense, not against self-defense. Like everything He said, it was said in context and He finished by directing us to do unto others as we would have them do unto us -- presumably to include defending one another from evil.*
Yes, I could send you to Grotius, but let's not fool about with one another: You've got neither the Latin nor the inclination. (Neither, I'll confess have I.) So let me direct you, instead, to Fx Hummel, shooter, troubadour, theology scholar, philosopher and -- best of all -- songwriter.
* Luke quotes Jesus this way in Chapter 6, verses 27 - 31:
“But to you who are listening I say: Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. If someone slaps you on one cheek, turn to them the other also. If someone takes your coat, do not withhold your shirt from them. Give to everyone who asks you, and if anyone takes what belongs to you, do not demand it back. Do to others as you would have them do to you."